Ultrasonic transducers have been available for quite some time and are particularly useful for non-invasive medical diagnostic imaging. Ultrasonic transducers are typically formed of either piezoelectric elements or of micro-machined ultrasonic transducer (MUT) elements. The piezoelectric elements typically are made of a piezoelectric ceramic such as lead-zirconate-titanate (abbreviated as PZT), with a plurality of elements being arranged to form a transducer. A MUT is formed using known semiconductor manufacturing techniques resulting in a capacitive ultrasonic transducer cell that comprises, in essence, a flexible membrane supported around its edges over a silicon substrate. The membrane is supported by the substrate and forms a cavity. By applying contact material, in the form of electrodes, to the membrane, or a portion of the membrane, and to the base of the cavity in the silicon substrate, and then by applying appropriate voltage signals to the electrodes, the MUT may be electrically energized to produce an appropriate ultrasonic wave. Similarly, when electrically biased, the membrane of the MUT may be used to receive ultrasonic signals by capturing reflected ultrasonic energy and transforming that energy into movement of the electrically biased membrane, which then generates a receive signal.
The MUT cells are typically fabricated on a suitable substrate material, such as silicon (Si). A plurality of MUT cells are electrically connected forming a MUT element. Typically, many hundreds or thousands of MUT elements comprise an ultrasonic transducer array. The transducer elements in the array may be combined with control circuitry forming a transducer assembly, which is then further assembled into a housing possibly including additional control electronics, in the form of electronic circuit boards, the combination of which forms an ultrasonic probe. This ultrasonic probe, which may include various acoustic matching layers, backing layers, and de-matching layers, may then be used to send and receive ultrasonic signals through body tissue.
Unfortunately, the substrate material on which the MUT elements are formed has a propensity to couple acoustic energy from one MUT element to another. This occurs because the substrate material is typically monolithic in structure and acoustic energy from one MUT element is easily coupled through the substrate to adjoining MUT elements. Therefore it would be desirable to have a way to fabricate a MUT substrate that reduces or eliminates the lateral propagation of acoustic energy.